The present invention relates to a circuit for controlling the rotation angle of a motor for driving a grating (diffraction grating) of a monochromator and, more particularly, to a circuit for controlling the rotation angle of a motor wherein the rotation angle of the motor is precisely detected in a wide range by a plurality of potentiometers disposed such that their phases are shifted from each other, thereby eliminating control errors generated in the unstable regions of the potentiometers.
In optical equipment such as a monochromator, a grating is disposed such that an angle of the grating can change with respect to the optical axis of an optical system. Light transmitted through an optical fiber or the like is spectroscopically analyzed to obtain intensities of light components of the respective wavelengths of the incident light.
In a conventional circuit for controlling a rotation angle of a motor, a rotation angle of the grating is detected and controlled by a single potentiometer. When a slider of a potentiometer is moved in one of the unstable regions F at the vicinity pause the resistor in the potentiometer, the variations in output from the potentiometer become great and disable the proper control of the angle of the grating, therefore, the use of the grating is limited. When the rotation angle of the potentiometer is controlled in synchronism with optical equipment (or a drive device such as a motor for driving the optical equipment), the ability of the circuit for detecting a small change in output (voltage) is influenced by noise or the like, thus inevitably limiting the detection capability.
In order to prevent this influence, a reduction gear is disposed between the potentiometer and the motor or the like for driving the optical equipment to decrease the rotation speed of the motor. The reduced rotation speed is detected by the potentiometer, so that the variable angle range of the grating or the like driven by the motor can be widened even if only one potentiometer having the unstable regions F is used.
However, in this case, a change in output from the potentiometer becomes small with respect to a corresponding change in the rotation angle of the grating or the like. The output from the potentiometer can be easily influenced by noise or the like, thus disabling highly precise angle control. In addition, the relationship between noise and control precision can also be applied to a multi-rotation potentiometer. More specifically, an output from this potentiometer can change only within the range of voltages applied thereto. Even in the multi-rotation potentiometer, a change in its output voltage becomes small due to the multi-rotation of the potentiometer. This change in output voltage cannot be substantially detected. Therefore, the multi-rotation potentiometer cannot be used to precisely control the rotation angle of the grating or the like.
However, if a reduction gear is not used to decrease the rotation speed of the motor or the like for driving the grating or the like in optical equipment so as to increase the change in output from the potentiometer with respect to the rotation, or if a reduction gear having a small reduction gear ratio is used, the rotation angle of the grating (or motor) can only be controlled in the region excluding the unstable regions F of the potentiometer, thus limiting the controllable range.